The invention relates to a method for determining the optimum operation control parameter values for a screw joint tightening process carried out by means of a power wrench controlled by a programmable control system in accordance with such determined optimum operation control parameter values.
Power wrenches controlled by programmable control systems are usually used in production work, assembly line operations in particular, where the accessibility of the power wrench and production efficiency have high priority. Programming of the control system and determining the optimum power wrench operation characteristics usually takes some time. The method usually practised today means that parameter values believed to be suitable for the intended tightening process are programmed into the control system, and a number of tightening operations are performed. The results are evaluated, the parameter values are adjusted, and another series of tightening operations is performed. In order not to lose too much time, this is usually done during the regular assembly work, mostly by slowing down or stopping the assembly line.
However, this type of iterating process to find the most favourable operation control parameter values is not only time consuming per se, but often results in readjustments and in some cases even replacement of damaged screw joints tightened during the programming procedure. The probability to find the optimum tightening parameter values by the very first more or less randomly chosen values is very small, which means that a number of xe2x80x9ctestxe2x80x9d tightenings have to be performed.
Alternatively, the power wrench could be lifted out of production for programming and test tightening. However, this means a highly undesirable reduction of the power wrench accessibility and production efficiency.
In order to speed up the programming procedure and substantially avoid readjustments of the screw joints tightened during the programming procedure, the invention provides a method for calculation of the optimum operation control parameter values for obtaining a tightening process having desired characteristics, including an acceptable tightening target accuracy. The method comprises a mathematical simulation of the tightening, including the use of parameter values characteristic for the actual type of screw joint as well as operation parameter values characteristic for the power wrench actually being used.
Further objects and advantages of the method according to the invention will appear from the following specification.
The invention is described below in further detail with reference to the accompanying drawings.